The Green Turtle's Tale
by Robin Meadows
Beautiful soup,
so rich and green,
Waiting in a hot tureen!
Who for such dainties would not stoop?
Soup of the evening, beautiful soup!
"Turtle Soup," as sung by the
Mock Turtle in Alice in Wonderland
When Christopher Columbus sailed through the Caribbean 500 years ago, his crew reportedly had to push green turtles aside with poles to clear passage for the ships. Columbus saw so many green turtles around three low islands 150 miles south of Cuba that he named them "Las Tortugas," Spanish for "The Turtles." The name didnt last long today these islands are known as the Caymans and neither did the turtles. While the Caymans once hosted the largest green turtle rookery in the Americas, today the nesting wild turtles are all but gone from these tropical islands. The story is much the same throughout the green turtles range, which once included tropical and subtropical waters worldwide.
Peoples
appetite for turtle productsfrom meat to eggs to jewelry
made of their beautiful shellshas just about done in green
turtles (Chelonia mydas) and the six other species of sea
turtles that ply the worlds oceans. But after years of decline,
there is now hope for the green turtle in the Caribbean. For
instance, green turtles are rebounding at Tortuguero Beach,
a 22-mile stretch of black sand in Costa Rica that is the
species most productive nesting site in the Western Hemisphere.
Efforts to conserve green turtles in the New World date back to at least 1620, when the Bermuda Assembly passed an act to prohibit killing turtles less than 18 inches across within five leagues of the islands. The assembly decried the many fishermen who "snatch & catch up indifferentlye all kinds of Tortoyses both younge and old, little and greate and soe kill, carrye awaye and devoure them to the much decay of the breed of so excellent a fishe."
The assemblys prediction that Bermuda was in "danger of an utter destroyinge and losse" of the green turtle has come true: Centuries of indiscriminate overharvesting has wiped out the islands huge nesting population.
That turtles remain at all in the Caribbean is due largely to the efforts of one manthe late Archie Carr, a University of Florida zoologist who was hailed as "the greatest conservation biologist of these troubled times" in a tribute written after his death in 1987. Carr began studying sea turtles in the 1950s. By then, the last significant green turtle breeding site in the Caribbean was Tortuguero, Costa Ricas black sand beach. Carr was appalled to discover that nearly every female that came ashore to nest was captured before she could lay eggs, then shipped to market and sold for meat.
Fearing that green turtles would soon die out at Tortuguero Beach and ultimately become extinct in the Caribbean, Carr wrote about the green turtles bleak outlook in The Windward Road, published in 1955. He proved to be as fine a writer as he was a biologist. A chapter of the book called "The Black Beach" won an OHenry Award for best short story.
More important, the book inspired New York newspaperman Joshua Powers to found The Brotherhood of the Green Turtle. In 1959, with funding from Tallahassee philanthropist John Phipps, the Brotherhood became the Caribbean Conservation Corporation (CCC), the first organization dedicated to preserving sea turtles. The CCC focused on protecting the nesting beach at Tortuguero and convincing the Costa Rican government to limit the harvest of adult green turtles.
That was a good start. But in the 1960s people knew so little about the turtles that they didnt know what else to do for them. Carr realized that the only way to save green turtles was to learn more about them. "To protect an animal, you have got to know where it isnot just once in a while but all the time," he noted in The Windward Road.
At that time, people knew that they nested only on particular beaches, and that the adults grazed on beds of sea grass and seaweed that grew along coastlines in tropical and subtropical waters around the world. What happened in between hatching and adulthood was anyones guess.
In Carrs day, the four big questions about green turtles were: Do they migrate? Do they return to nest on the beach where they were born? How do they navigate? And, finally, where do the hatchlings go?
Carr himself confirmed, through an extensive tagging program, that green turtles do migrate. Since Carr began his work in the mid-1950s, more than 35,000 nesting females have been tagged. These studies showed that turtles that nest at Tortuguero Beach disperse to feeding grounds all over the Caribbean and beyond, migrating as far as Florida, Mexico, and Venezuela.
Studies inspired by Carr have also confirmed that green turtles return to nest on the beach where they were born, according to DNA studies by Brian Bowen of the University of Florida and former-Carr student Anne Meylan of the Florida Marine Research Institute in Melbourne Beach. Swimming up to 200 miles per day, both males and females return to mate in offshore waters. Females then crawl ashore, where they dig a hole and lay about 100 ping pong ball-sized eggs about two feet below the surface of the sand. Females lay up to six clutches over the course of the summer nesting season and then skip a few years before returning to the beach to nest again.
"Their tremendous migrations are some of the longest and most dramatic in the marine world," says David Owens of Texas A&M University in College Station, Texas. The green turtles that graze off the coast of Brazil embark on the most spectacular voyage of all. They swim about 1,200 miles through the open ocean to nest on Ascension Island, which lies roughly halfway between Brazil and Africa.
A mere seven miles across, Ascension Island offers a perfect example of the third sea turtle mystery: How do they find such a tiny piece of land at such a tremendous distance? Sea turtles can raise their heads only a few inches out of the water and have poor vision. Even if they could see well, few landmarks exist in the open ocean. "Animals that make scheduled convergences on small oceanic islands are doing some pretty far-out navigating," said Carr in his 1967 book, So Excellent a Fishe.
Studies
of hatchlings are now revealing how green turtles perform
such feats of navigation. "Hatchlings seem to know exactly
where theyre going," says Blair Witherington of the
Florida Marine Research Institute in Melbourne Beach. "Theyre
in a frenzythey pop out of their nests, run to the water,
and swim like crazy."
The hatchlings need to be fast because the two-minute dash to the sea is short but perilous. Birds, crabs, and other predators snatch up and devour as many of the two-inch hatchlings as they can.
How do the baby turtles know which way to run? They emerge at night, when the cool air and sand make dehydration less likely, and find the sea by heading for the brightest horizon. Under natural conditions, the ocean is always brighter at night because water reflects more light than sand. When beaches have street lights or brightly lit buildings, however, hatchlings can get confused and run inland, where they dehydrate and die.
Once in the water, hatchlings swim continuously for about two days. They try to reach open ocean as soon as possible because sharks and other predators are abundant in nearshore waters. So many hatchlings are eaten on their way to the open ocean that fewer than one percent are estimated to survive to sexual maturity.
How do the hatchlings know which way to swim? At first, they swim into the waves, which parallel the beach when close to shore. Farther from shore, however, wave direction no longer correlates with the direction of shore. Biologists believe that hatchlings then switch to navigating using a biological magnetic compass. Studies by Ken Lohmann of the University of North Carolina at Chapel Hill show that sea turtles can indeed detect and orient according to magnetic fields.
Lohmanns work suggests that sea turtles can sense both the angle and intensity of the Earths magnetic field, which vary depending on where you are on the surface of the planet. The angle of the magnetic field gives the latitude of a given position and the intensity gives the longitude. "Knowing where you are is essential to navigation," says Witherington. "The second thing you need to know is where to go. We think hatchlings somehow record the latitude and longitude of their natal beach."
Studies of hatchlings are also answering the fourth green turtle mystery: Where do the baby turtles go? Once again, Carr was basically right. "The most likely idea seems to be that the hatchlings for a time become plankton, that they drift more or less passively in the open sea," he wrote in So Excellent a Fishe.
After swimming out to the open ocean, hatchlings get swept into convergence zones, areas where surface waters are driven into each other by currents or the wind. Convergence zones are also known as weedlines because they collect sargassum, a brown floating seaweed. Weedlines range from being relatively small and ephemeralnarrow strips a few hundred feet long that last only while the wind blowsto being vast and essentially permanent. Where the Gulf Stream swirls up to the central north Atlantic, weedlines can be a dozen miles long and several miles wide, and so persistent that the area is called the Sargasso Sea.
For the few hatchlings that survive the gauntlet of the predator-filled beach and nearshore waters, weedlines are a haven. Few predators lurk in weedlines while prey abounds. Unlike adults, green turtle hatchlings are carnivorous and feed on all kinds of creatures from worms to crustaceans to insects. The baby turtles just float around and eat and grow until they reach eight to ten inches, which typically takes several years. Then they undergo a dramatic change, switching to a vegetarian diet of sea grass and algae and abandoning the weedlines for the adult grazing grounds.
Carr didnt muse about a fifth sea turtle mystery, presumably because he knew the answer. But generations of children have wondered why the mock turtle in Alice in Wonderland is always crying. It turns out that sea turtles really do cry. They drink seawater and have to get rid of all that extra salt somehow. So they concentrate it in glands behind their eyes and shed large, salty tears.
While Carrs understanding of green turtles basic biology was right, he would be happy to know that so far his prediction of the species demise has been wrong. Far from dying out at their nesting beach in Costa Rica, the turtles are making a comeback. This is because Costa Rica has made green turtle protection a priority. The country has restricted the harvest of eggs and adults since the late 1950s and early 1960s, and recently banned the sale of green turtle eggs and meat altogether. In addition, in 1976 Costa Rica established Tortuguero National Park to protect the turtles nesting beach.
Carr began keeping track of the number of nests built on Tortuguero Beach in 1971 and that work is being continued today by another of his former students, Karen Bjorndal, and other biologists at the Archie Carr Center for Sea Turtle Research in Gainesville, Florida. The surveys show that the number of nests has tripled over the last quarter of a century, rising from about 15,000 in 1971 to 50,000 in 1996.
Promising as this is, the researchers caution that the increase in the number of nests tells us only that mature females are doing well. Hatchlings in the Sargasso Sea could be dwindling and we would not know it for decades because green turtles take 30 to 50 years to reach sexual maturity. "Drastic changes in the survival of early life stages...would not be apparent on the nesting beach for many years," wrote Bjorndal and her colleagues in the February 1999 issue of Conservation Biology.
On a smaller scale, green turtles are also rebounding in Florida, which holds 90 percent of the sea turtle nesting beaches in North America. In 1989 Congress established the Archie Carr National Wildlife Refuge on a 20-mile-long beach along Floridas Atlantic coast, between Melbourne and Wabasso. So far, the U.S. Fish & Wildlife Service (USFWS) has purchased nearly two-thirds of the remaining 9.3 miles of undeveloped beach in the refuge at a cost of $60 million, which came from government conservation programs and the Mellon Foundation. But without more funding, the USFWS may lose the chance to buy the rest of the beach because coastal Florida is being developed so rapidly. "Supporters of the Refuge are literally in a race against time to acquire the best remaining parcels of undeveloped land," says the Caribbean Conservation Corporation.
Since the refuge was established, the number of green turtle nests has risen from about 200 to nearly 2,000 in 1998. The refuge is also one of the worlds most important nesting sites for loggerhead turtles, and the number of loggerhead nests there has increased from about 8,000 to about 22,000 in 1998.
While green turtles are doing well in the few sites where they are protected, they are declining in most of the rest of their range, says Marydele Donnelly, Washington, D. C.-based Program Officer of the IUCN Marine Turtle Specialist Group. The most serious threat worldwide is the harvest for meat and eggs. The biggest green turtle nesting population anywhere is in Australia, where island beaches just a couple of miles long can be crammed with tens of thousands of nests. But more than 100,000 green turtles are harvested each year for meat in the Australasian region. And eggs from nearly all the green turtle nests are harvested in Indonesia and Malaysia.
The best bet for protecting the green turtle is eliminating the markets for their meat, eggs, and shells, says Donnelly. Ways to do this include expanding international agreements to ban sea turtle trade, and encouraging people to rely on the turtles to attract ecotourism income rather than to provide food.
While ranching has been proposed for crocodiles and other types of wildlife harvested for their meat, this approach is simply not feasible for sea turtles, say both Donnelly and the Florida Marine Research Institutes Witherington. "Ranching is a bad idea because sea turtles are hard to raise due to their complicated life historytheyre not like cattle," says Witherington. "Ranching could create markets for sea turtles, which could increase wild catchings. Wild sea turtles will be less safe in a world hungry for turtle soup, eggs, and shell jewelry. I hope the world decides to want turtles swimming around more than turtle soup."
The second greatest threat to sea turtles is accidental deaths caused by fishing gear. Commercial shrimp trawling accounts for most of these fatalities. More than 55,000 sea turtles used to drown in shrimp trawling nets each year in waters off the southeastern U.S. alone. Since the early 1990s, shrimp nets in U.S. waters have been required to have Turtle Excluder Devices, which are like trapdoors that let turtles escape. But not all fishermen comply with this federal requirement, and these devices are not widely used elsewhere in the world.
Longline fisheries are a growing threat to sea turtles. Intended to catch bottom-dwelling fish, the 300-foot lines have six-inch hooks that also snag sea turtles. "There are billions of hooks on longlines in the open ocean and the fishery is growing rapidlyabout 15 percent per year in the Indian Ocean," says the IUCNs Donnelly. She advocates closing longline fisheries seasonally in waters where sea turtles are abundant.
Another threat to sea turtles is marine pollution, particularly in the weedlines where hatchlings feed. In addition to collecting sargassum, baby turtles, and the creatures they eat, weedlines collect tar balls, plastic, and other debris. And, unfortunately, hatchlings appear to be indiscriminate eaters. "One of the scary things we find is that hatchlings have tar and plastic in their gutsabout half have eaten tar and about a fifth have eaten plastic," says Witherington. "The number of sea turtles eating tar is high and alarming." Tar can make their jaws stick together and both tar and plastic can plug up their digestive systems, which can ultimately kill the turtles. Analysis of tar yields a fingerprint of its component hydrocarbons, and Witherington has found that most of the tar in hatchling guts comes from bunker oil, which has not yet been refined. The most likely sources are oil tankers that flush sludge out of their tanks at sea instead of at reclamation facilities in port, he says. Because flushing at sea is already illegal, the answer is stronger enforcement. The same is true of plastics dumped at sea. "All this happens way out in the open ocean so sea turtles could be dying in great numbers and wed never know it," says Witherington. "This is our first glimpse of what might be going on out there."
Marine pollution may also hurt sea turtles indirectly by weakening their immune systems, making them susceptible to disease and infection. For example, scientists are finding more and more green turtles with fibropapillomas, skin tumors that can reach ten inches in diameter. Turtles with fibropapillomas growing on or near their eyes, flippers, or mouths can have such a hard time seeing, swimming, or eating that they die. In some places, 80 percent of young green turtles are afflicted.
Fibropapillomas are a relatively new threat to green turtles. "You solve one suite of problems and turn around to another suite," says Donnelly.
"The good part of the story is that people are realizing that we have to conserve sea turtles by regional management. This is a breakthroughits not just what you do here in one place that counts with migratory species."
Archie Carr couldnt have said it better himself.
ZooGoer 28(5) 1999. Copyright 1999 Friends of the National Zoo. All rights reserved.
